A spark plug is mounted to, for example, an internal combustion engine and used to ignite air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole, a center electrode inserted into the axial hole, a metallic shell provided externally of the outer circumference of the insulator, and a ground electrode provided on the front end surface of the metallic shell and adapted to form a spark discharge gap in cooperation with the center electrode. When the metallic shell and the insulator are assembled together, generally, a stepped portion provided on the inner circumferential surface of the metallic shell and a stepped portion provided on the outer circumferential surface of the insulator butt against each other via a sheet packing made of metal (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-303661).
In a combustion chamber, carbon is generated as a result of incomplete combustion of air-fuel mixture or the like and may accumulate on the surface of a portion (leg portion) of the insulator exposed to air-fuel mixture and combustion gas. When carbon progressively accumulates on the surface of the leg portion and covers the surface of the leg portion, current may leak from the center electrode to the metallic shell via carbon accumulated on the leg portion, or a spark discharge may be generated between the insulator and the metallic shell, potentially hindering the generation of a normal spark discharge across the spark discharge gap. Particularly, in recent years, in direct-injection engines and the like employed for improvement of fuel economy and output, carbon is more likely to adhere to the insulator, so that the above problem is more likely to occur.
The present invention has been conceived in view of the above circumstances, and an object of the invention is to provide a spark plug which can reliably prevent adhesion and accumulation of carbon onto the insulator for improving resistance to fouling.